The green strength, or more specifically the lack of green strength of synthetic rubber is a significant hinderance to the use of synthetics in many applications. While green strength is a somewhat elusive physical property to quantify, as used in this specification it refers to those physical properties of an uncured elastomer which permit molding or construction of a multicomponenet article without subsequent "flow" or distortion prior to curing. Natural rubber commonly possesses these properties and it is partially for this reason that natural rubber is still used in many compound recipes, particularly those used in making large articles such as truck tires. Green strength is usually measured by stress/strain curves of unvulcanized compounds with the yield point and the ultimate or breaking tensile being the values used to quantify the amount of green strength.
Because synthetic materials commonly used as substitutes for natural rubber, e.g., styrene-butadiene rubber (SBR), polybutadiene, and the like, lack green strength, the substitution is not entirely satisfactory. To overcome this deficiency, a variety of compounds may be added to the compounding recipe in order to increase green strength.
One group of compounds which have been particularly useful in the improvement of green strength are the carboxyl-containing compounds. These compounds, however, must be copolymerized in order to be effective. Copolymerization has been found to be particularly difficult in standard emulsion polymerizations constituting the preferred processes for manufacture of SBR, polybutadiene and the like. The problem lies in the fact that standard emulsion polymerizations are carried out in a basic medium, that is, in an aqueous phase having a pH of between about 9 and 10, and under these conditions, the normally used carboxylic acids are neutralized into water soluble salts which are incorporated into the polymer backbone to a much smaller degree than are the acids themselves. This is due both to the fact that the salts are much less reactive than the acids, and that the salts tend to stay in solution in the aquaeous phase.
In the prior art, this problem was dealt with through the expedient of creating an acidic medium in which the carboxylic acids remain in the acid state and therefore less soluble. This process, however, carries with it a host of problems such as the need for corrosion resistant vessels and piping and the need for cationic surfactants. These changes from the standard emulsion polymerization process add considerably to the cost of manufacture.
The instant invention provides for the use of novel carboxyl-containing compounds which unexpectedly can be used in basic mediums in emulsion polymerizations while at the same time are sufficiently incorporated into the polymer so as to effect improved green strength. The compounds of this invention are half esters of dicarboxylic acids such as for example, maleic and fumaric acids. The ester moiety must be a rather long chained hydrocarbon which imparts to the half ester a degree of water insolubility. The half ester is thus less susceptible to attack by the basic medium of the standard emulsion polymerization process.
Prior art references which teach the use of carboxyl-containing compounds include U.S. Pat. No. 3,898,983 to Brancaccio which relates to the improvement of green strength of a polyisoprene through reaction with maleic acid. U.S. Pat. No. 3,897,403 to Yamauchi, et al, relates to the use of maleic anhyride in a reaction with synthetic cis-1,4 polyisoprene. Neither of these patents teach the use of water insoluble half esters of a dicarboxylic acid such as maleic acid or acid anhydride.
French Pat. No. 2,215,429 relates to the use of small amounts of various carboxylic acids with synthetic polymers such as polybutadiene and SBR to improve the green strength. There is no mention of the use of hydrophobic half esters of dicarboxylic acids such as fumaric or maleic.
U.S. Pat. No. 3,429,952 to Nordsiek, et al, discloses the use of unsaturated carboxylic acids to increase the tear resistance of cis-polybutadiene. It is not pertinent however, in that it teaches the neutralization of the carboxylic acids to form acid salts which cannot be utilized in the instant invention.
U.S. Pat. No. 2,880,186 to Barth, teaches the use of unsaturated carboxylic acids to produce strong elastic films having increased tear resistance. While this patent refers to emulsion polymerizations, an acid aqueous medium must be used. U.S. Pat. No. 3,475,362 to Romanick, et al, describes the use of carbon-containing compounds in rubber based adhesives. There is no mention made of the use of half esters of dicarboxylic acids.
U.S. Pat. No. 4,254,013 to Friedman, et al, describes improvements in green strength of elastomer blends obtained through the use of ionogenic compounds in the polymer chains. This application does not however teach the use of half esters of dicarboxylic acids.
Two articles in Rubber Chemistry and Technology disclose that unsaturated carboxylic acids may be copolymerized with olefins and dienes, Brown and Gibbs, Rubber Chemistry and Technology, Volume 28, page 938 (1955) and Brown, Rubber Chemistry and Technology, Volume 30, page 1347 et seq., (1957). These articles specifically refer to the problems associated with emulsion polymerizations of butadiene, SBR and other polymers in which carboxylic acids are incorporated. There is no suggestion of the use of half esters of dicarboxylic acids in emulsion polymerizations so as to allow polymerization in basic aqueous mediums.